(a) Field of the Invention
The present invention relates to a fine particulate polishing agent comprising fine particles of a solid solution composed of single-crystal ceric oxide and silicon dioxide and fine particles of silicon dioxide; a stable slurry polishing agent comprising the fine particulate polishing agent; a method for producing the slurry polishing agent; and a method for producing a semiconductor device which makes use of the slurry polishing agent. More specifically, the present invention pertains to a fine particulate polishing agent and a slurry polishing agent which are suitable for use in various applications which require extremely high surface polishing accuracy, for instance, polishing of a variety of glass materials which comprise, as principal components, silicon dioxide, polishing of semiconductor materials such as silicon wafers and polishing of interlayer insulating films of silicon dioxide in semiconductor device-manufacturing processes, and which are suitably used for removing a superficial SiO.sub.2 film through polishing while using an Si.sub.3 N.sub.4 layer as a stopper for polishing since they have a high polishing rate for SiO.sub.2 and they have a very low polishing rate for Si.sub.3 N.sub.4 (i.e., a high selective polishing ratio of SiO.sub.2 to Si.sub.3 N.sub.4), as well as a method for producing these polishing agents and a method for producing semiconductor devices using the polishing agents.
(b) Description of the Prior Art
In the techniques wherein the surfaces of vitreous materials are polished to a mirror finish, in particular, those which should satisfy the requirement for extremely planar surface, the surface thereof has in general been polished using colloidal silica. Incidentally, the polishing with colloidal silica causes a problem of low polishing rate. Therefore, when a large amount of a material must be removed through polishing, the material is polished by a two-stage process. Firstly, it is polished with a polishing agent which mainly comprises ceric oxide and can ensure a high polishing rate and secondly, it is polished with colloidal silica, as disclosed in, for instance, Japanese Un-Examined Patent Publication No. Sho 64-40267. Such a two-stage polishing process requires the use of additional polishing installations and a superabundant time for switching over to the polishing with colloidal silica. This accordingly leads to a substantial reduction in the productibility.
There has been increasingly a demand for polishing techniques which can ensure surface planarity and therefore, it is very important to develop a novel polishing agent which supersedes colloidal silica, i.e., a polishing agent capable of achieving surface planarity comparable to or superior to that attained by the colloidal silica polishing agent and capable of ensuring a polishing rate comparable to that observed for the ceric oxide polishing agent.
The applicant of this invention has proposed, in Japanese Patent Application Serial No. Hei 6-272761, a polishing agent for use in semiconductor device-manufacturing processes, which comprises ceric oxide having an average particle size of not more than 0.1 .mu.m. The fine particulate polishing agent can ensure a good polishing rate even if the particles thereof have a very small particle size on the order of not more than 0.1 .mu.m, unlike the conventional ceric oxide polishing agents and the polishing rate is substantially higher than that achieved by using the silica polishing agents.
If quartz glass is polished with the polishing agent comprising ceric oxide particles having such a small particle size, the glass surface thus polished has a quite planar surface considerably superior to that achieved by the conventional ceric oxide polishing agents. Nevertheless, the polishing rate is found to be almost comparable to that achieved by the conventional ceric oxide polishing agents. More specifically, the use of ceric oxide fine particles ensures the polishing rate comparable to that observed for the conventional ceric oxide polishing agents, while ensuring the surface planarity approximately identical to that observed for the colloidal silica polishing agents.
However, the foregoing fine particulate polishing agent of ceric oxide causes the following problems. First, the ceric oxide particles show poor dispersibility in a medium (water) and this in turn makes it difficult to supply polishing agent fine particles uniformly to a polishing machine and a subject to be polished. Moreover, such a polishing agent shows high resistance which requires high torque to rotate the substrate. Therefore, the use thereof becomes a cause of various troubles of the subject when it has a small thickness and any increase in the polishing pressure during polishing cannot be admitted when the polishing machine used has poor rigidity.
On the other hand, when a polishing agent comprising colloidal silica is used, the polishing rate is quite susceptible to the pH value of the polishing liquid used and for this reason, the pH thereof should be maintained at a level of not less than 10 in order to ensure a polishing rate excellent in reproducibility. The polishing liquid having a pH of not less than 10 is very difficult for operators engaged in the polishing to handle the same and such a liquid is harmful to the operators.
Moreover, when a semiconductor device is produced using a multi layer interconnection circuit, the surface thereof must be processed into a predetermined shape. The polishing has widely been used as an effective technique for flattening or planarizing the surface of an insulating film. In particular, in the processing of semiconductor devices, there has been used the CMP (Chemical Mechanical Polishing) method as a surface-flattening method. In this CMP method, it has been known that the polishing rate is accelerated by the polishing effect through chemical reactions. In the polishing and surface-flattening processes used for manufacturing shallow trench isolation in semiconductor devices, a surface film is removed through polishing while using Si.sub.3 N.sub.4 as a stopper for polishing, but this operation requires the use of a polishing agent which has a sufficiently high ratio of the surface film-polishing rate to the stopper-polishing rate (i.e., a high selective polishing ratio of the surface film to the stopper) or a polishing agent which can effectively remove the surface film, but can hardly remove the stopper.